Recording capacities of a hard disk drive are constantly increasing, and recording densities of a magnetic recording medium included therein are also continuously increasing. In a conventional longitudinal magnetic recording method, thermal fluctuation has become a major problem to increase recording densities, and technical innovations, to commercialize perpendicular magnetic recording type magnetic medium, which are new recording methods to solve the above problem, have just recently become available. Recording densities are increasing nonstop at an annual rate of about 50%. This is due to the constant advancements in all kinds of technical fields, not just due to the above mentioned shift in recording methods. For example, concerning HDI (Head Disk Interface) technology, the flying height of a magnetic head is decreasing every year, and a further decrease in the distance between a magnetic layer of a magnetic recording medium and a magnetic head (magnetic spacing) is demanded. As one means of decreasing the magnetic spacing, decreasing the thickness of a protective layer formed on the magnetic layer, and that of a lubricating layer, is demanded.
The protective layer plays a role of protecting the magnetic layer and must have high corrosion resistance so that the magnetic layer, of which major component is a metal material, does not corrode by reacting with moisture in the air or with corrosive gas. Additionally the protective layer must be durable against the running of a magnetic head. This is to prevent damage of the magnetic layer when the magnetic head, which is flying above the magnetic recording medium at a relatively high-speed with a small magnetic spacing, contacts with the magnetic recording medium due to a malfunction.
The lubricating layer plays a role of allowing the magnetic head to run smoothly and stably above the magnetic recording medium. A very thin lubricating layer is formed on the protective layer, and corresponds an outermost layer of the magnetic recording medium.
An element to determine the performance of the protective layer is the denseness of the protective layer. Therefore for the protective layer, a carbon layer having high denseness is normally used, and even among the carbon layers, a diamond-like carbon (DLC) layer is normally used. The DLC layer has an sp3 bonding where carbon elements densely bond with one another with a strong bonding force, and is therefore ideal in terms of corrosion resistance and durability, as mentioned above.
To form a DLC layer, a plasma CVD (Chemical Vapor Deposition) method using hydrocarbon gas as a raw material is widely used. According to the plasma CVD method, a plasma state is generated by applying energy to a compound gas containing atoms constituting the film, whereby active ions and radicals are generated, and cause chemical reactions, and a thin film is formed as a result. As the thickness of the protective layer decreases, coatability is affected, hence earnest research to maintain corrosion resistance and durability by further increasing denseness are ongoing. In the case of a carbon layer, increasing the sp3 bonding ratio is critical, and a method to generate high density plasma must be selected. In J. Robertson, Diamond and Related Materials, Vol. 3, (1994) pp. 361-368, it is stated that the sp3 bonding ratio can be increased by optimizing the energy of carbon ions generated by the plasma. In other words, optimizing film deposition conditions is also important.
The plasma CVD method is classified into the following types depending on the plasma generation method and the plasma density to-be-generated. For example, the plasma density is about 1011 cm−3 in the CCP (Capacitively Coupled Plasma) type, about 1010 cm−3 to 1011 cm−3 in a thermal filament type, about 1011 cm−3 to 1012 cm−3 the ECR (Electron Cyclotron Resonance) type or in the ICP (Inductively Coupled Plasma) type, and about 1012 cm−3 or more in the MEICP (Magnetically Enhanced Inductively Coupled Plasma) type, where a DC magnetic field is added to the ICP method.
It is known that the DLC layer has high water repellency and has a large contact angle with respect to water (hereafter simply called “contact angle”). Water repellency becomes higher as the film has higher corrosion resistance and denser structure. Therefore by using this characteristic, a method of obtaining high corrosion resistance by specifying the protective layer using a specific contact angle was proposed. For example, see Japanese Patent Application Laid-open Nos. S61-222024, H08-167138 and H09-237415 (No. 1 to 3). Japanese Patent Application Laid-open No. S61-222024 is characterized in that the contact angle is 75° or more, and Japanese Patent Application Laid-open No. H08-167138 is characterized in that the contact angle is 60° or more. Moreover, Japanese Patent Application Laid-open No. H09-237415 is characterized in that the contact angle is 80° or more.
A molecule of a lubricant includes a main chain having a lubrication function, and a terminal group having a polarity to bond with the protective layer. If the water repellency of the protective layer is high, an interaction, such as hydrogen bonding, chemical bonding and polar interaction, between the terminal group of the lubricant and the protective layer becomes weak, and a sufficient amount of the lubricant cannot be coated under normal coating conditions. If the lubricant is insufficient when a dedicated cleaning head or cleaning tape scans the surface of the magnetic recording medium in the surface cleaning step, which is performed before testing the flyability of the magnetic head in the manufacturing steps of the magnetic recording medium, then strong friction with the protective layer is generated due to insufficient lubrication, and the surface of the magnetic recording medium is scratched and the medium becomes useless. Even if coating conditions are changed and the coating film thickness is secured in this state, the ratio of the thickness of the non-bonded lubricant with respect to the total coating film thickness increases, and therefore when the magnetic head flies and travels, the non-bonded lubricant flows into a peripheral area, due to the wind pressure that is generated during the flying of the magnetic head, which results in a drop in lubricity in an area where the magnetic head traveled, or results in an unstable flying of the magnetic head caused by the transfer of the lubricant to the magnetic head.
As a solution to this problem, a treatment to decrease the repellency of the surface of the protective layer before coating the lubricant has been proposed. For example, Japanese Patent Application Laid-open No. 2001-266328 discloses that the contact angle of the protective layer with respect to water is decreased to 10° to 30° by treating the surface of the protective layer, down to a very shallow area from the surface, using nitrogen plasma. The nitrogen plasma treatment is a method of generating plasma in a chamber where nitrogen gas is introduced, which allows active nitrogen ions and nitrogen radicals to react with the surface of the protective layer, so that the repellency of the surface is decreased by the nitrogen absorbed into the surface of the protective layer.    Patent Document 1: Japanese Patent Application Laid-open No. S61-222024    Patent Document 2: Japanese Patent Application Laid-open No. H8-167138    Patent Document 3: Japanese Patent Application Laid-open No. H9-237415    Patent Document 4: Japanese Patent Application Laid-open No. 2001-266328    Non-patent Document 1: J. Robertson, Diamond Related Materials, Vol. 3, (1994), pp. 361-368
At the moment, there is a demand to decrease the thickness of the protective layer to about 2.5 nm. Further, as design guidelines for hard disk drives in the future, a 1 Tb/in2 recording density and 6.5 nm magnetic spacing are proposed, which means that the protective layer must be even thinner. Concerning the magnetic spacing, the thickness of the protective layer of the magnetic head is about 2 nm, the distance between the outermost surface of the magnetic head and the outermost surface of the magnetic recording medium (flying height of the magnetic head) is about 2 nm, and the thickness of the lubricating layer of the magnetic recording medium is about 0.5 nm to 1 nm, therefore it is expected that the thickness of the protective layer of the magnetic recording medium is 2 nm or less.
If such an extremely thin protective layer is required, various problems are generated which never existed before. When the protective layer is treated with nitrogen plasma, deterioration of the protective layer caused by the nitrogen plasma treatment must be minimized.
If the sp3 bonding ratio of the DLC layer is increased to improve durability, water repellency increases along with the increase of the sp3 bonding ratio, and bondability between the lubricant and the protective layer deteriorates.
A possible solution to this problem is to constitute the protective layer of the magnetic recording medium by two layers made of different materials. For example, the protective layer is constituted by two layers, where silicon is used for the lower layer, and DLC is used for the upper layer. However if different types of materials are bonded, the protective layer may become unstable since warping tends to occur to the protective layer because the physical properties, such as the thermal expansion coefficients, of the materials are different, and inter-facial mismatch effects are generated.